diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef4b16a4..d9e0af4a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 - Add `assert_have_been_called_nth_with` for verifying arguments on the Nth invocation of a spy
 - Add `assert_string_matches_format` and `assert_string_not_matches_format` with format placeholders (`%d`, `%s`, `%f`, `%i`, `%x`, `%e`, `%%`)
 - Add JSON assertions: `assert_json_key_exists`, `assert_json_contains`, `assert_json_equals` (requires `jq`)
+- Add duration assertions: `assert_duration`, `assert_duration_less_than`, `assert_duration_greater_than`
 
 ### Changed
 - Split Windows CI test jobs into parallel chunks to avoid timeouts
diff --git a/docs/assertions.md b/docs/assertions.md
index 55b4c54b..b37cbfac 100644
--- a/docs/assertions.md
+++ b/docs/assertions.md
@@ -1270,6 +1270,57 @@ function test_failure() {
 ```
 :::
 
+## assert_duration
+> `assert_duration "command" threshold_ms`
+
+Reports an error if `command` takes longer than `threshold_ms` milliseconds to execute. Uses the framework's portable clock internally.
+
+::: code-group
+```bash [Example]
+function test_success() {
+  assert_duration "echo hello" 500
+}
+
+function test_failure() {
+  assert_duration "sleep 2" 1000
+}
+```
+:::
+
+## assert_duration_less_than
+> `assert_duration_less_than "command" threshold_ms`
+
+Reports an error if `command` takes `threshold_ms` milliseconds or more to execute. Stricter than [assert_duration](#assert-duration) which allows equal values.
+
+::: code-group
+```bash [Example]
+function test_success() {
+  assert_duration_less_than "echo hello" 500
+}
+
+function test_failure() {
+  assert_duration_less_than "sleep 2" 1000
+}
+```
+:::
+
+## assert_duration_greater_than
+> `assert_duration_greater_than "command" threshold_ms`
+
+Reports an error if `command` completes in `threshold_ms` milliseconds or less. Useful for verifying that a command takes at least a minimum amount of time.
+
+::: code-group
+```bash [Example]
+function test_success() {
+  assert_duration_greater_than "sleep 1" 500
+}
+
+function test_failure() {
+  assert_duration_greater_than "echo hello" 5000
+}
+```
+:::
+
 ## bashunit::fail
 > `bashunit::fail "failure message"`
 
diff --git a/src/assert_duration.sh b/src/assert_duration.sh
new file mode 100644
index 00000000..7e33f0b9
--- /dev/null
+++ b/src/assert_duration.sh
@@ -0,0 +1,84 @@
+#!/usr/bin/env bash
+
+function bashunit::duration::measure_ms() {
+  local command="$1"
+
+  local start_ns
+  start_ns=$(bashunit::clock::now)
+
+  eval "$command" >/dev/null 2>&1
+
+  local end_ns
+  end_ns=$(bashunit::clock::now)
+
+  local elapsed_ms
+  elapsed_ms=$(bashunit::math::calculate "($end_ns - $start_ns) / 1000000" | awk '{printf "%.0f", $1}')
+
+  echo "$elapsed_ms"
+}
+
+function assert_duration() {
+  bashunit::assert::should_skip && return 0
+
+  local command="$1"
+  local threshold_ms="$2"
+
+  local elapsed_ms
+  elapsed_ms=$(bashunit::duration::measure_ms "$command")
+
+  if [ "$elapsed_ms" -gt "$threshold_ms" ]; then
+    local test_fn
+    test_fn="$(bashunit::helper::find_test_function_name)"
+    local label
+    label="$(bashunit::helper::normalize_test_function_name "$test_fn")"
+    bashunit::assert::mark_failed
+    bashunit::console_results::print_failed_test "${label}" "${threshold_ms}" "to complete within (ms)" "${command}"
+    return
+  fi
+
+  bashunit::state::add_assertions_passed
+}
+
+function assert_duration_less_than() {
+  bashunit::assert::should_skip && return 0
+
+  local command="$1"
+  local threshold_ms="$2"
+
+  local elapsed_ms
+  elapsed_ms=$(bashunit::duration::measure_ms "$command")
+
+  if [ "$elapsed_ms" -ge "$threshold_ms" ]; then
+    local test_fn
+    test_fn="$(bashunit::helper::find_test_function_name)"
+    local label
+    label="$(bashunit::helper::normalize_test_function_name "$test_fn")"
+    bashunit::assert::mark_failed
+    bashunit::console_results::print_failed_test "${label}" "${threshold_ms}" "to complete within (ms)" "${command}"
+    return
+  fi
+
+  bashunit::state::add_assertions_passed
+}
+
+function assert_duration_greater_than() {
+  bashunit::assert::should_skip && return 0
+
+  local command="$1"
+  local threshold_ms="$2"
+
+  local elapsed_ms
+  elapsed_ms=$(bashunit::duration::measure_ms "$command")
+
+  if [ "$elapsed_ms" -le "$threshold_ms" ]; then
+    local test_fn
+    test_fn="$(bashunit::helper::find_test_function_name)"
+    local label
+    label="$(bashunit::helper::normalize_test_function_name "$test_fn")"
+    bashunit::assert::mark_failed
+    bashunit::console_results::print_failed_test "${label}" "${threshold_ms}" "to take at least (ms)" "${command}"
+    return
+  fi
+
+  bashunit::state::add_assertions_passed
+}
diff --git a/src/assertions.sh b/src/assertions.sh
index c612afb8..2cd50ee1 100644
--- a/src/assertions.sh
+++ b/src/assertions.sh
@@ -2,6 +2,7 @@
 
 source "$BASHUNIT_ROOT_DIR/src/assert.sh"
 source "$BASHUNIT_ROOT_DIR/src/assert_arrays.sh"
+source "$BASHUNIT_ROOT_DIR/src/assert_duration.sh"
 source "$BASHUNIT_ROOT_DIR/src/assert_files.sh"
 source "$BASHUNIT_ROOT_DIR/src/assert_folders.sh"
 source "$BASHUNIT_ROOT_DIR/src/assert_json.sh"
diff --git a/tests/unit/assert_duration_test.sh b/tests/unit/assert_duration_test.sh
new file mode 100644
index 00000000..61b3e609
--- /dev/null
+++ b/tests/unit/assert_duration_test.sh
@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# shellcheck disable=SC2329
+
+function test_successful_assert_duration_within() {
+  assert_empty "$(assert_duration "sleep 0" 1000)"
+}
+
+function test_successful_assert_duration_within_fast_command() {
+  assert_empty "$(assert_duration "echo hello" 500)"
+}
+
+function test_unsuccessful_assert_duration_exceeds_threshold() {
+  assert_same \
+    "$(bashunit::console_results::print_failed_test \
+      "Unsuccessful assert duration exceeds threshold" \
+      "1000" "to complete within (ms)" "sleep 1")" \
+    "$(assert_duration "sleep 1" 1000)"
+}
+
+function test_successful_assert_duration_less_than() {
+  assert_empty "$(assert_duration_less_than "sleep 0" 1000)"
+}
+
+function test_unsuccessful_assert_duration_less_than() {
+  assert_same \
+    "$(bashunit::console_results::print_failed_test \
+      "Unsuccessful assert duration less than" \
+      "100" "to complete within (ms)" "sleep 1")" \
+    "$(assert_duration_less_than "sleep 1" 100)"
+}
+
+function test_successful_assert_duration_greater_than() {
+  assert_empty "$(assert_duration_greater_than "sleep 1" 500)"
+}
+
+function test_unsuccessful_assert_duration_greater_than() {
+  assert_same \
+    "$(bashunit::console_results::print_failed_test \
+      "Unsuccessful assert duration greater than" \
+      "5000" "to take at least (ms)" "echo hello")" \
+    "$(assert_duration_greater_than "echo hello" 5000)"
+}